We investigate the transcriptional control of T cell development and function, and specifically of the developmental steps at which T cell precursors differentiate into CD4 or CD8 lineage cells. Most T cells recognize peptide antigens presented by class I (MHC-I) or class II (MHC-II) classical Major Histocompatibility Complex molecules. T cell recognition of MHC-peptide complexes is aided by two surface glycoproteins called coreceptors: CD4, which binds MHC-II, or CD8, which binds MHC-I. Coreceptor expression on mature T cells is mutually exclusive and strongly correlates with both MHC specificity and functional differentiation. That is, the general rule is that MHC I-specific T cells are CD4CD8+ and cytotoxic (CD8 cells), whereas MHC II-specific T cells are CD4+CD8and helper (or regulatory) (CD4 cells). This double correspondence is essential to the proper function of the immune system and is established in the thymus, where CD4 and CD8 T cells emerge as separate lineages from precursors expressing both CD4 and CD8 (double positive, DP). Deciphering the transcriptional regulatory networks that decide and maintain CD4-CD8 lineage differentiation is the target of our current research. Our efforts during the last two years have focused on the role of two zinc finger transcription factors, Gata3 and Zbtb7b (also known as cKrox or Thpok) in the differentiation of CD4 T cells. Gata3 is required for the development of CD4 T cells and for their proper effector differentiation after antigen encounter. As to Zbtb7b, we previously reported that it inhibits CD8 and promotes CD4-differentiation in the thymus, while, in parallel, the group of Dietmar Kappes at Fox Chase identified a spontaneous mutation in the Zbtb7b gene that causes an almost complete block of CD4 cell development. However, while both factors were known to be required to generate CD4 cells, their respective roles in this process had so far remained unclear. To address this question, we generated mice carrying null alleles of Zbtb7b, or a GFP-based bacterial artificial chromosome transgene to monitor Zbtb7b expression. We also obtained mice deleting Gata3 in DP thymocytes, generated in the laboratory of Bill Paul (NIAID). Using these animals, we made the following findings. First, disruption of either Gata3 or Zbtb7b blocks the development of CD4 T cells at an early stage of their differentiation, before they commit to the CD4 lineage (i.e. they lose the ability to adopt a CD8 fate). Second, the requirement for Gata3 precedes that for Zbtb7b; notably, Gata3 is needed for Zbtb7b expression, whereas the reverse is not true. We found that Gata3 binds within a region of the Zbtb7b locus critical for Zbtb7b expression, suggesting that Gata3 directly activates Zbtb7b transcription. Third, Gata3 is needed for Zbtb7b to promote CD4 differentiation, but not for Zbtb7b to inhibit CD8 differentiation. These findings, recently published in Nature Immunology (Wang et al., 2008), led us to propose that Gata3 acts as a specification factor for the CD4 lineage, notably by enabling the expression of CD4-lineage specific genes, whereas Zbtb7b serves as a commitment factor, by preventing the expression of CD8-linage genes and thereby sealing the CD4 fate. This conclusion fits with that of ongoing experiments assessing the function of Zbtb7b in mature T cells. In a recent report (Jenkinson et al., 2007), we have expressed Zbtb7b by retroviral transduction into mature (peripheral) CD8 T cells in which it is normally not expressed. We found that Zbtb7b represses the expression of CD8 and of characteristic cytotoxic genes, including those encoding perforin and granzyme B. Conversely, we found a more modest induction of helper-specific genes (including those encoding IL-2 and IL-4) in Zbtb7b-transduced CD8 T cells. These observations support our hypothesis that Zbtb7b is primarily a repressor of the CD8-cytotoxic gene expression program. We are currently addressing this issue using loss-of-function mouse models. These findings defined Zbtb7b as a major commitment factor for the CD4 lineage, and suggest that similar gene networks control CD4-CD8 both during and after intrathymic T cell differentiation, an issue that is currently addressed in the laboratory.